Realistically I don't think there is (one thing) someone could do that would cause one to fall in love forever. Whatever you might enjoy could be done by 10 people but you're not likely to fall in love with all ten of them!
We fall in love with who a person is. This would include a variety of things they do as well as things they don't do. Chemistry also plays a major role. One of the key differences between "relationship love" and (parental, sibling, and friendship love) is our romantic desire for our significant other.
It's not about what is done but who does it and how you feel about them. If you don't like a person there is nothing they can do that would cause you to fall in love.